Evaluation of a suite of simple fire indices for the 2014 Fire Season in the Pacific Northwest

There are numerous indices for wildland fire that reflect the weather conditions relevant to fire behavior or fire danger. A partial list includes the Fosberg fire weather index, the Haines Index, the National Fire Danger Rating System's components, and the Canadian Fire Weather Index (and its components). Computational complexity and required data for these indices vary, and few if any can be computed manually. These indices tend to be complex functions of the input variables, making it difficult to discern just what aspect of the weather is likely to cause problems. Simple indices have the advantage that they are easier to calculate and easier to evaluate, and hopefully less likely to be misused. Based on this reasoning, we sought to create and evaluate a set of simple fire weather indices.

A suite of simple fire weather indices (SFWIs) are evaluated for 15 wildfires in the northwestern United States, during the summer of 2014. Three specific SFWIs were evaluated, and for each SFWI, four variations of the input variables were tested. Each of these twelve permutations relied on temperature, humidity and wind. The variations for each index included: surface values of the input variables; average values over a layer of fixed depth; average values over the local thermodynamic equilibrium height; and maximum value of each input variable within a layer of fixed depth. Each index-variation is compared to daily fire growth to evaluate index performance. The individual components of the indices (temperature, humidity and wind) are also compared to fire growth, since any index that does not out-predict an input component is not worth computing.